The present invention relates generally to digital image processors which digitalize an image signal representing a predetermined font to express a digital image corresponding to the predetermined font on matrically-arranged picture elements by using the luminance of each picture element (abbreviated pixel hereinafter), and more particularly to a digital image processor which determines the luminance of each pixel based on an antialiasing process by using a matrix having matrix elements corresponding to the picture elements, on which matrix the predetermined font is expressed. Therefore, the luminance of each picture element is determined by calculating the luminance of a corresponding matrix element, and the luminance of each matrix element is calculated in accordance with an antialiasing process. The term "antialiasing process", as used herein, means a process which proportions the luminance of each matrix element to an area ratio of an area to be covered by the predetermined font in a corresponding matrix element for an area of the corresponding matrix element in order to smooth jagged edges of the digital image. Hereupon, the predetermined font is generally referred to as an outline vector font according to the Outline Vector Format. In the Outline Vector Format, an outline of a font is relatively numerically expressed, and thus the inside demarcated by the outline is drawn with straight and curved lines, as described in "PIXEL", No.16, January, 1984, Scan-line Conversion (2) concerning Polygons, pp.136-141, published by the Information Center. The antialiasing process is generally used for Disk Top Publishing (called DTP for short).
With the recent spread of DTP, as described in "The Journal of the Institute of Image Electronics Engineers of Japan", Vol.18, No.3, June 1989, the use of an outline vector font as a font for the DTP has also spread. The outline vector font represents uniquely designed letters and figures expressed by straight and curved lines representing the outline thereof instead of by dot patterns. The outline vector font originally includes little aliasing when expressed on pixels as a digital image. The term, "aliasing", as used herein, means that an edge of the font is jagged. But if an antialiasing process is further performed on the digital image, it includes less aliasing. The term "an antialiasing process", as used herein, means a process to removing aliasing, that is, smoothing a jagged edge of the digital image. There are at least two kinds of antialiasing process, as described in "Practice Computer Graphics Basis and Application" edited by Fujio Yamaguchi and translated by SEIKO Electroengineering Inc. Electronic Division. One kind thereof infinitesimally reduces each pixel. But there is a limit how much a pixel can be reduced. Accordingly, a description will now be given of another kind of antialiasing process, with reference to FIGS. 1(A), 1(B), 2, 3(A), 3(B), 3(C) and 3(D).
The antialiasing process, to which the present invention is to be applied, is a process to shade the jagged edges off by means of proportioning the luminance of each pixel to an area ratio of area to be covered by the predetermined font in a pixel for the pixel area, as shown in FIGS. 1(A) and 1(B). FIG. 1(A) shows jagged edges, which look like four tiers, and FIG. 1(B) shows corresponding smoothed edges, thus look like a parallelogram, after the antialiasing process is performed to the jagged edges. To be concrete, as shown in FIG. 2, a luminance value E representing the luminance of a pixel R is defined as follows: ##EQU1## hereupon, A.sub.1 ; area of an image part R.sub.1, A.sub.2 ; area of a non-image part R.sub.2, E.sub.1 ; a luminance value of the pixel R before drawing the image, E.sub.2 ; a luminance value of the image part R.sub.1, k; A.sub.2 /(A.sub.1 +A.sub.2), an area ratio of the area of the non-image part for the area of the pixel R The term "an image part", as used herein, means a group of pixels which include a part of a digital image, and the term "a non-image part" means another group thereof exclusive of the image part. In addition, pixels which belong to the image part and are to be traversed by an outline of a font are classified as "an image edge part", hereinafter.
However, the above antialiasing process has the following disadvantage in that the image edge part is emphasized. Hereupon, the term "the image edge part is emphasized", as used herein, means that the luminance of the image edge part is the highest among that of the image part, the non-image part, and the image edge part, so that the jagged edge is not successfully smoothed. Basically, the image edge part is not emphasized when the luminance of the image edge part is lower than that of the image part. But, even when the luminance of the image edge part is higher than that of the image part, if the luminance of the image edge part is lower than that of the non-image part, it may be considered that the image part is not emphasized. When expressing an outline vector font on sixteen pixels, the outline of the font seem to divide the pixels into two kinds of parts; an image part and a non-image part, as shown in FIG. 3(A). But, since a pixel cannot be divided into two kinds of parts, each of the pixels are actually partitioned into three kinds of parts; an image part, an image edge part, and a non-image part, as shown in FIG. 3(B). When it is assumed that a luminance value E.sub.2 is 100 and a luminance value E.sub.1 is 0, a luminance value of the image part is 100, because A.sub.2 is 0 in the quotation (X), and a luminance value of the non-image part is 0. On the other hand, a luminance value E of the image edge part is calculated from the quotation (X) as follows, since k is found to be 1/2 from FIG. 3(B): EQU E=(1/2)*0+(1/2)*100=50
However, since the antialiasing process fixes the luminance value of each pixel on the basis of the previous luminance value E.sub.1, there is a case where the luminance of the image edge part is the highest of the three kinds of parts of the pixels. For example, if a font having a luminance value 10 and shaped the same as FIG. 3(B) is superimposed on that in FIG. 3(B), it is expected that the luminance value of the image edge part will not be the highest of the above three kinds of parts, for example, 5, as shown in FIG. 3(C). But, actually, the luminance value of the image part is 10 and the luminance value of the non-image part is 0, whereas the luminance value E of the image edge part is calculated by E.sub.1 is 50, E.sub.2 is 10, k is 1/2: EQU E=(1/2)*50+(1/2)*10=30
Thus, contrary to the above expectation, the luminance value of the image edge part becomes the highest of the above three kinds of parts so that the image edge part is emphasized. This disadvantage may occur when deleting a previously-drawn image by means of a luminance value 0.